1. Field of the Invention
The present invention relates to an InP single crystal substrate provided for fabrication of laser diodes, photodiodes and other optical communication devices, and more particularly, to an InP single crystal substrate in which the occurrence of defects in the surface of an epitaxial layer is held to a low level.
2. Description of the Related Art
InP single crystal substrates are used as substrates for fabricating laser diodes and photo diodes used in optical communications. In order to fabricate laser diodes and photo diodes, it is necessary to form a thin film consisting of a group III through V compound semiconductor and so forth by epitaxial growth on an InP single crystal substrate polished to a mirrored state. Epitaxial growth technologies using a liquid phase method or vapor phase method are used to grow this thin film. Recently, it has become necessary to reduce the thickness of the epitaxial layer grown on the InP single crystal substrate in order to get higher frequency response of these elements. However, as a result of decreasing the thickness of the epitaxial layer, the quality of the InP single crystal substrate has a considerable effect on the performance of the epitaxial layer.
One example of this is the defect of protrusions appearing on the surface of epitaxial wafers referred to as hillocks. Although the size of the hillocks varies according to the epitaxial conditions, they have a diameter of about 20 xcexcm and height of about 5 nm.
In order to eliminate these hillocks, various accommodations have been adopted such as changing the composite ratio of the III-V compound semiconductor epitaxial layer, adjusting the substrate temperature during epitaxial growth, and giving a slight off angle to the surface orientation of the single crystal substrate. For example, Japanese Unexamined Patent Application, First Publication No. 07-193007 discloses the inclining of the surface orientation of a substrate only by an angle of 0.03-0.08xc2x0 from an orientation of  less than 100 greater than  to eliminate hillocks followed by performing epitaxial growth while restricting the substrate temperature to 700-800xc2x0 C.
However, in the case of preventing the occurrence of hillocks by changing the epitaxial growth conditions, there was the problem of it being difficult to obtain an epitaxial layer offering the desired level of performance. In addition, in the case of preventing hillocks by giving an off angle to an InP single crystal substrate, there was the problem of microscopic defects appearing in the epitaxial layer thereby causing poor uniformity of the film.
In consideration of the above problems, the object of the present invention is to provide an InP single crystal substrate that inhibits the occurrence of hillocks on an epitaxial wafer surface.
The inventors of the present invention achieved the present invention as a result of earnest efforts and studies to solve the above problems. Namely, the present invention is:
[1] an InP single crystal substrate characterized in that, the oxygen atom concentration in the InP single crystal substrate is within the range of 1xc3x971017 atoms/cm3 to 1xc3x971018 atoms/cm3;
[2] the InP single crystal substrate according to [1] wherein, the off angle of the surface orientation of the InP single crystal substrate is less than xc2x10.03xc2x0;
[3] the InP single crystal substrate according to [2] wherein the surface orientation of the InP single crystal substrate is  less than 100 greater than ;
[4] a production process of the InP single crystal substrate according to any of [1] through [3] comprising: growing InP single crystal using a raw material containing indium oxide and/or phosphorous oxide followed by processing the grown single crystal on a substrate;
[5] a production process of the InP single crystal substrate according to any of [1] through [3] comprising: adding indium oxide and/or phosphorous oxide to an encapsulant in the form of B2O3, growing InP single crystal according to the liquid encapsulated Czochralski method, and then processing the grown single crystal on a substrate;
[6] an InP single crystal substrate fabricated using the production process according to [4] or [5]; and,
[7] a laser diode or photo diode fabricated using the InP single crystal substrate according to [1] through [3] or [6].
[8] an epitaxial wafer for the laser diode or photo diode fabricated using the InP single crystal substrate according to [1] through [3] or [6].